1. Field of the Invention
The present invention relates to a so called ATM (Asynchronous Transfer Mode) network which is one of the most promising high-speed, broadband communication network, and more particularly, to a cell loss rate estimation, a call admission control, and a buffer/link capacity designing suitable for such an ATM network.
2. Description of the Background Art
Recently, the ATM network has been attracting much attentions as one of the most promising high-speed, broadband communication network. The ATM is a mode of transferring various information such as audio information, video information, and other data in terms of blocks of a fixed length called cells, as such information is generated. The ATM network is a communication network using this ATM as a transfer mode.
In the ATM network, as shown in FIG. 1, a request for setting up a call (call generating cell) have to be made from a terminal to the network (an exchanger) first, and then only when the call set up is admitted by the network, the cells (user information) corresponding to this call are sent to the network, until this call is finished (releasing cell).
Such an ATM network generally comprises, as shown in FIG. 2, a number of ATM exchangers 1 mutually connected by links 2; and a number of terminals 3, each of which is connected to one of the ATM exchangers 1.
Each ATM exchanger 1 comprises, as shown in FIG. 3, a switch unit 1--1 for receiving cells from an input link; a call admission control unit 1-2, connected to the input link, for controlling admission to be granted to each incoming call; and output buffers 1-3, connected to an output link, for temporarily storing the cells received by the switch unit 1--1.
The call admission control unit 1-2 receives a call set up request cell containing information concerning a call set up, which is normally a top cell of a group of cells corresponding to the call, and determines whether this call is admissible or inadmissible, i.e., whether the network can accommodate this call or not, according to the information concerning a call set up. Namely, as shown in FIG. 4, the call admission control unit 1-2 comprises a traffic parameter receiving unit 1-21 for receiving prescribed set of parameters related to the call set up from the call set up request cell; a cell loss rate estimation unit 1-22 for estimating the cell loss rate according to the parameters received by the traffic parameter receiving unit 1-21, by means of a convolution operation unit 1-23; and an admission judging unit 1-24 for judging whether or not to grant an admission to this call according to the estimated cell loss rate.
Thus, when the admission is granted by the admission control unit 1-2, the cells of the call arriving to the ATM exchanger 1 from the input link are exchanged at the switch unit 1--1, and then sent out to the output link through the output buffers 1-3. Here, the output buffers 1-3 have a finite capacity, so that when the number of arriving cells exceeds the given capacity of the output buffers 1-3, the excessive cells are simply discarded. Such a discarding of the excessive cells deteriorates the quality of service (grade of service). Thus, the admission control unit 1-2 have to estimate the cell loss rate, i.e., what fraction of all the cells received is likely to be discarded, and determine the admissibility of the call according to whether the estimated cell loss rate is within a predetermined tolerable range for which a satisfactory quality of service can be maintained.
This operation of admission control unit 1-2, in which whether the admission is to be granted to the call or not, i.e., whether the cell loss rate resulting from the admission of this call is still within the tolerable range or not, is determined according to the cell loss rate estimated on a basis of the specified parameters, is called a call admission control.
In general, in a course of the call admission control, it becomes necessary to estimate and evaluate the cell loss rate of each output buffer in a case the requesting call is admitted, on a basis of the traffic parameters of the call requesting a set up, the traffic parameters of the connected calls, and the measurement results for the number of cells produced by the connected calls, in order to judge whether the cell loss rate of each output buffer in a case the requesting call is admitted is lower than the predetermined cell loss rate reference value.
Here, the traffic parameters contain the information concerning the number of cells to be produced by the call, such as an average bit rate and a peak bit rate, for example. The average bit rate is a rate obtained by dividing a bit length of one cell by an average cell production interval, and the peak bit rate is a rate obtained by dividing a bit length of one cell by a shortest cell production interval. Such traffic parameters have to be specified by the user beforehand, so that it is preferable to select easily specifiable quantities as these traffic parameters.
In the ATM call admission control, the estimation and evaluation of the cell loss rate in a case the requesting call is admitted have to be performed at high-speed, because it is necessary to respond very quickly to the call set up request.
The cell loss rate is also indispensable for the ATM network designing in which a buffer capacity or a link capacity suitable for the network is dimensioned.
In this regard, in a conventional network such as a conventional telephone network, a number of communication lines (a maximum number of calls that can be set up simultaneously) is uniquely determined from the amount of offered load between the exchangers and the call loss rate, from which the link capacity can be determined easily. For example, for 100 communication lines, the link capacity has to be 64 Kb/s.times.100=6.4 Mb/s.
However, in the ATM network, even when the maximum number of calls that can be simultaneously connected is given, the link capacity cannot be determined. This is because, in the ATM network, the cell loss rate for the cells produced by the call set up becomes a complicated function of a number of connected calls, statistical characteristics of arrival of cells, the output buffer capacity, and the link capacity.
In the ATM network designing, a number of connected calls, statistical characteristics of arrival of cells, the prescribed cell loss rate reference value, and the quality reference value such as the maximum tolerable delay are given, and the output buffer capacity and the link capacity are dimensioned from these information. Thus, in the ATM network designing, it is necessary to estimate how the cell loss rate of each output buffer changes when the buffer capacity and the link capacity are varied while the number of connected calls and the statistical characteristics of arrival of cells are fixed, for each output buffer of each link.
Here, the statistical characteristics are preferably those related to the lower moments for the cell arrival interval or the number of cells arriving within a given period of time, as they are easy to measure.
As for the statistical characteristics related to the cell arrival process, the statistical characteristics concerning the traffic parameters such as a distribution of the specified average bit rates may be used, rather than the characteristics directly related to the actual cell arrival process, so that it is preferable that the input of the designing apparatus is capable of reflecting the specified values.
Moreover, the designing apparatus should be constructed without a specialized procedure such as a modelling of the arrival process which calls for a human operation, after the statistical characteristics related to the arrival of cells are given.
Now, the conventional method for estimating the cell loss rate in the ATM network can be classified into two categories.
The first method is that which has been achieved with the application to the call admission control in mind, the detail of which had been reported by Suzuki et al. in "A simple and burst-variation independent measure of service quality for ATM traffic control", the international teletraffic congress, seventh specialist seminar, New Jersey, October, 1990. In this method, a probability distribution F of a number of cells.times.(bit/s) resulting for a given instance due to the traffic parameters of each call is derived at each output link, and by using the link capacity C (bit/s), the estimated value for the cell loss rate is obtained as: ##EQU1## where E[x] denotes an average value of x.
The second method is that which utilizes a so called queuing theory, in which the cell loss rate for each buffer is obtained from the distribution of the cell arrival intervals.
Of these two method, the first method completely ignores the output buffer capacity so that it has the drawbacks of:
i) being inapplicable to the ATM network designing including the designing of the output buffer capacity; and
ii) being unable to ascertain whether the estimated value for the cell loss rate is greater or smaller than the actual cell loss rate, so that the estimated value cannot be evaluated as to whether the condition with respect to the prescribed cell loss rate reference value is satisfied or not.
As for the second method, it requires a complicated analytical process, so that it has the drawbacks of:
i) being unable to adapt the high-speed processing;
ii) being based on the quantities which are both difficult to measure in the ATM network in which a large number of cells are transferred at the high-speed, as well as difficult for the user to specify beforehand as the traffic parameters, such as the higher moments for the cell arrival interval or the correlations;
iii) being requiring a so called modelling step which calls for a human operation; and
iv) calls have to be classified into a small number of categories.